Generally, a fuel cell is a device which converts the energy of a chemical reaction into electricity. Fuel cells differ from batteries in that fuel and oxidant are stored external to the cell, which can generate power as long as the fuel and oxidant are supplied. A fuel cell produces an electromotive force by bringing the fuel and oxidant in contact with two suitable electrodes separated by an electrolyte. An electrolyser is a device with the opposite function of a fuel cell. It converts electrical energy to chemical energy in the form of hydrogen and oxygen. In a polymer electrolyte fuel cell, a fuel such as hydrogen gas, is introduced at one electrode where it dissociates on the electrocatalytic surface of the negative electrode (anode) to form protons and electrons, as elucidated in equation 1. The electrons pass into the conductive structure of the electrode, and there from to the external electrical circuit energized by said fuel cell. The protons formed by dissociation of the hydrogen at the first electrode—pass through the electrolyte to the second electrode. Simultaneously, an oxidant, such as oxygen gas or air, is introduced to the second electrode where it is adsorbed on the electrocatalytic surface of the positive electrode (cathode) and is electrochemically reduced to form a surface oxide species by electrons having transversed the external electrical circuit energized by the fuel cell. This surface oxide reacts with protons from the electrolyte to form water, the product of the net reaction. The water desorbs from the electrode and leaves the cell in the cathode. Some of the formed water, being in condensed form, remain in the cathode and the hygroscopic membrane. The half cell reactions for a hydrogen consuming fuel cell at the two electrodes are, respectively, as follows:H2′2H++2e−  (1);½O2+2H++2e−′H2O  (2)
Connecting the two electrodes through an external circuit causes an electrical current to flow in the circuit and withdraws electrical power from the cell. The overall fuel cell reaction, which is the sum of the separate half cell reactions written above, produces electrical energy and heat.
Although some applications may make use of a single cell, fuel cells are in practice often connected in a series to additively combine the individual cell potentials and achieve a greater, and more useful, potential. The cells in a given series can be connected directly, with opposing faces of a single component in contact with the anode of one cell and the cathode of an adjacent cell, or through an external electrical linkage. A series of fuel cells, referred to as a fuel cell stack, are normally equipped with a manifold system for the distribution of two gases. The fuel and oxidant are directed with manifolds to the correct electrodes, and cooling is provided either by the reactants or by a cooling medium. Also within the stack are current collectors, cell-to-cell seals, and other components. The stack and associated hardware make up the fuel cell module.
In fuel cells which use a solid polymer electrolyte, the membrane acts as the electrolyte as well as a barrier for preventing the mixing of the reactant gases. A PEM fuel cell is described in greater detail in Dhar, U.S. Pat. No. 5,242,764, which is incorporated herein by reference. Much research and development has been devoted to improving the power-to-weight ratio for proton exchange membrane (“PEM”) fuel cells. Most of this research has involved increasing the power per unit volume of relatively heavy stacks.
The polymer electrolyte electrochemical device of the present invention is a small device. Unlike the situation for stationary or transportation applications (1-200 kW), the main motivation for developing small polymer electrolyte fuel cells (micro to hundreds of watts) does not reside in environmental benefits but in foreseeable improved technical characteristics compared to the established technologies which are, chiefly, primary and secondary batteries. Also, materials cost is a lesser barrier than in transportation applications since batteries are relatively expensive. The expected advantages of Polymer Electrolyte Fuel Cells (PEFC) against batteries are a higher energy density (Wh g-1) and no recharging time.
Hitherto, PEFC's have been developed chiefly for large cells where the benefits of having certain regulations (temperature, reactant flows and humidity) are not outweighed by the implied weight and electrical consumption of the associated ancillary components (cooling system, compressors and fans, humidifiers). More recently, efforts have been made to reduce the stack weight by replacing the heavy carbon elements with thinner and lighter, metal elements. However, these units were designed for large scale applications, some on the order of about 30 kW, and, therefore, require the same stack ancillary equipment mentioned above. Furthermore, the ancillary equipment included with the stack in these systems has been designed to operate efficiently at the kilowatt level. Scaled down versions of these systems have been attempted in applications that require much less power, such as within the range between about 50 and about 150 Watts. However, these systems are not well suited for stack outputs in the tens or hundreds of watts, since the rotating components, such as pumps and compressors, do not scale down well. As a result, even small scale systems of this design are too heavy for many small applications, such as for portable applications and personal use.
Therefore, the most important objective for portable and personal applications is perhaps not Watts per unit volume but Watts per unit weight, i.e. W/g and W/cm3.
Small fuel cells must be designed to work with minimized control. The design has naturally shifted from stacks to planar cells, since planar cells offer enhanced heat removal and air access to the cathode. Any planar configuration implies in turn a mixed conductor/insulator pattern for serial connections. Serial connection between planar cells can be made in two ways. The first, which is often referred to as the banded design, consists of having cathodes arranged on either side and anodes on the opposite side and each cathode being connected to the anode of the next adjacent cell. The connection may be made by creating breaches in the central area of the membrane or by leading the current aside the active area beyond the membrane edge and making the connection there. The latter choice avoids cutting out through the membrane and in so doing avoids the fastidious tightening of each anode separately. The second way of making a serial connection of planar cells is often referred to as the flip-flop design, and involves construction of two cell-house plates, each having cathodes and anodes alternated along its surface. A cathode of one cell is then electrically connected to an anode of the next cell. Passive PEFCs do require membranes having small resistance, regardless of the design chosen for the in-plane serial connection, even if the water originates only from the fuel cell reaction. This fact calls for thin membranes as long as fuel crossover is not a concern. If the PEFC works on average at current densities of 200 mA cm-2 or more, fuel crossover is not a concern and the membrane thinness is limited only by its mechanical integrity.
Three types of materials and their associated technologies have so far been investigated for small PEFCs:
(i) Miniaturization of conventional PEFC design with graphite or stainless steel plates for the current collectors and cell housing;
(ii) Silicon technology; either patterning of conductive/nonconductive path on silicon wafers or development of methods to create in the wafer a complex architecture of porous silicon layers on top of channels for the reactant gases;
(iii) Printed Circuit Board (PCB) technology; use of a thin layer of copper on electrically insulating composite materials.
The present invention is not restricted to these technologies, since it only requires a gas supplying support plate, which can be made from any sufficiently gas tight material (e.g. metals, plastics or even paper or paper composites).
There is a whish to improve and to simplify fuel cells. Some attempts have been made, that involve partial replacement of the clamping means by adhesive bonding. One method is described in US 2004/0161655, which discloses the assembly of a electrochemical stack by adhesively bonding the non-active perimeter of a membrane electrode assembly to the perimeter frame of one side of a bipolar plate, using a desired number of membrane electrode assemblies and bipolar plates. A thin layer of curable or thermoplastic adhesive is placed on the sealing areas, and the cell frames and membrane electrode assemblies are pressed together until the adhesive is fully cured and bonds the cell frames and the membrane electrode assemblies together. In this method care must be taken to ensure that the membrane and electrode assemblies are oriented properly so that the cathode side of one membrane and electrode assembly faces the cathode side of a bipolar grid or bipolar plate to which the membrane and electrode assembly is being bonded. Furthermore, adhesive must be carefully applied to the perimeter of any features on the bipolar element, such as, for example, a flow field, a manifold, a channel and combinations thereof to provide the necessary fluidic seal keeping reactant fluids, cooling fluids, or heating fluids confined to their respective areas. Moreover, cell stack produced by this method will still need a clamping force, especially for larger cells.
Therefore, there is a need for a lightweight fuel cell system that provides an improved power density and eliminates much of the ancillary equipment. There is also a need for high performance fuel cells or electrolyser that are simple to produce and convenient to handle. The object of the present invention is thus to provide an improved electrochemical device that can function as a fuel cell or an electrolyser. Another object of the present invention is to provide an electrochemical device which allows easy replacement of the active component.
The above objects are achieved in a patent application PCT/SE2005/001514, published as WO 2006/041397 where the fuel cell is formed by adhesively attaching an active component (current collectors, GDLs and MEA) to a support plate. The invention described below refers to important improvements of that invention and also improving other planar configuration fuel cells.